The increasing use of computer data communications over the past several years has spawned a variety of network architectures to support requirements for distributed processing. Developed by various R&D groups,<sup>1-3</sup>by the common carriers,<sup>4-4</sup>by minicomputer and mainframe manufacturers,<sup>7,8</sup>and by the vendors of traditional communications hardware,<sup>9,10</sup>these new architectures represent alternative means to similar ends. This article provides a framework for understanding existing and forthcoming systems, focusing particular attention on the impact of evolving requirements and technologies.